DNP enhancement of 20 obtained following microwave irradiation of an antimony-doped silicon wafer (2.5 × 1017 cm−3) at 1.4 K. Note that the hyperpolarized signal is opposite to the thermally polarized signal. The inset shows the basic DNP experiment.
Build up of 29Si polarization in antimony-doped silicon (2.5 × 1017 cm−3). The enhancement obtained with the 100 mW source is clearly power-limited. With the 1 W microwave source the hold time of the cryostat shortened from 4 to 2 h. A 5º pulse was used to monitor the polarization in the DNP experiments while a 90º pulse was used in the thermal equilibrium situation .
DNP of P-doped silicon (N D = 3 × 1017 cm−3) at 1.1 K. The sample is placed in a tuned TE011 cylindrical resonator during the microwave irradiation, and then physically moved approximately 1 in. to the center of the RF coil where the polarization is measured. The DNP signal (one average) was recorded from a 2.8 mg piece of silicon wafer. The thermal signal (eight averages) was recorded from a sample that also contained an additional 2.1 mg wafer fragment (total weight 4.9 mg). The ratio of the measured signal intensities is 17.4 ± 5.8, indicating a DNP enhancement of 244 ± 81.
ESR spectra of phosphorus- and antimony-doped silicon crystals obtained at X-band at a temperature of 3.4 K . The signal from the phosphorus-doped sample was obtained under rapid-passage conditions. The spectrum shows the two hyperfine resolved lines, the broad center line, and additional peaks that arise from donor clusters. The signal from the antimony-doped sample did not quite satisfy the rapid-passage conditions, resulting in some lineshape distortions. The antimony-doped sample shows the presence of 14 hyperfine resolved lines as well as the broad center line. We did not resolve the cluster peaks in this sample.
Frequency dependence of DNP enhancement for both phosphorus- and antimony-doped silicon, acquired with the 100 mW Gunn source.
Schematic illustration of a two-donor cluster, showing the overlap of the electron orbitals with a number of 29Si nuclei.
Eigenstates of the three-spin Hamiltonian shown in Eq. (4), where α = cos θ/2, β = sin θ/2, γ = cos ϕ/2, δ = sin ϕ/2, tan ϕ = − J ⊥/(2δ + A/2), and tan θ = − J ⊥/(2δ − A/2). The dashed black lines show the allowed ESR transitions that are not excited by the applied microwaves. The solid blue lines show the allowed ESR transitions that are resonant with the applied microwave irradiation. The dotted red line indicates the hyperfine-mediated cross-relaxation path leading to DNP. The energy levels shown in dark green correspond to the symmetric manifold while the levels shown in light green correspond to the asymmetric manifold. We have labeled the states such that , and . The figure also shows the offset ESR frequencies ω − ω e as a function of the exchange coupling for the lower electron spin manifolds for δ = 0 and 60 MHz. For δ = −60 MHz the curves are almost identical to those for δ = +60 MHz. We used A = 4 MHz, D = 0 MHz, and ω n = 20 MHz.
Parameters obtained by fitting the data in Fig. 2 to Eq. (10), using A = 2π × 1 MHz. The amplitude of the DNP data was multiplied by 11.5 to account for the smaller flip angle pulsed used.
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